1. Field of the Invention
The present invention relates to the turbine rotor of a radial or mixed flow type turbine that is used in turbochargers and the like; the present invention especially relates to the rear side surface geometry of the turbine rotor.
2. Background of the Invention
In the turbine rotors of the turbochargers used for vehicle engines, marine engines and the like, when turbine rotor has a great deal of rotational inertia (moment of inertia), the start-up characteristic regarding the engine speed as well as the charging air pressure is deteriorated, as shown in FIG. 7 that shows a response characteristic regarding the engine system in which the turbocharger is included; a result, a time lag regarding the response characteristic is generated between a time point when the inputted gas condition is changed and a time point when the engine speed as well as the charging air pressure is kept in a steady state.
Accordingly, as a method to reduce the rotational inertia of the turbine rotor, an approach to arrange the geometry of the turbine rotor by removing or cutting a part of the blade is known.
For instance, an approach as shown in FIG. 8 is known; thereby, the original shroud line (i.e. the tip end side line regarding the rotor blade) 05 is lowered toward the rotation axis, to an alternative line so that the height of the trailing edge 03 of the blade 01 is reduced, Further, an approach as shown in FIG. 9 is known; thereby, the thickness of the blade 01 is reduced to the thickness of the blade 01′; or the position of the shroud line as well as the leading edge 07 is lowered so that the turbine itself is down-sized.
In a case of the approach where the height of the trailing edge 03 of the blade 01 is reduced or the approach where the thickness of the blade is reduced as described above, however, the approach may cause efficiency deterioration or spoil the strength requirement. In addition, in a case of an approach where the downsized turbine rotor is used, the turbocharger has to bypass a part of pressurized charging air, the to-be-bypassed flow rate reaching the difference between the flow rate at the maximum torque point and the flow rate at the maximum output point; thus, there may be a difficulty that the efficiency of the whole system is reduced.
Hence, it has been proposed to provide a recess part on the rear surface side of the turbine rotor so that the rotational inertia is reduced without changing the blade geometry, the recess part being formed as a concave part of the rear surface by removing a part of the mass of the rotor (hub) on the rear surface side.
For instance, Patent Reference 1 (JP1998-54201) discloses a turbine rotor as depicted in FIG. 10; thereby, on the side surface 016 of the hub 015 on which a plurality of blades 013 of the turbine rotor 011 is provided, an annular recess part 017 is formed, the depth direction of the recess being parallel to the rotation axis direction of the turbine rotor.
Further, Patent Reference 2 (JP1988-83430) discloses a turbine rotor as depicted in FIG. 11; thereby, on the side surface 025 of the hub 024 on which a plurality of blades 022 of the turbine rotor 020 is provided, a plurality annular recess parts 026 is formed, the depth direction of the recess being parallel to the rotation axis direction of the turbine rotor. The number of the recess parts 026 is thereby four; each annular recess part is formed along the hoop direction as well as the rotation axis direction regarding the turbine rotor; the recess part in a cross-section whose plane includes the rotation axis is formed in a rough approximation of a triangle shape.